Firstly, the 3 second rule was invented to help guys get over approach fears. The idea behind this is that if you take longer than 3 seconds to approach you start to over think things and it puts you off. Seeing any truth to that yet?
Secondly, I refer to the above statement. You're over analysing the situation. Have a plan, keep it simple, and stick to it. To put those worries of yours to rest, she gets hit on all the time but she doesn't get asked out. By going the extra step you are being different. Don't tip any more than what you usually would, because you don't want to look like you're trying to butter her up so she'll reciprocate interest in you. That's some bad shit right there, so don't do it.
As for getting what you want from her, simply get her attention before you leave. She'll ask if everything is OK, to which you reply, "Yeah everything's great, I just wanted to know what time you finish so I can take you out". That little line is courtesy of Gunwitch, and as much as I love the guy, I'm more inclined towards my own version, "I was just thinking, you and me should get together some time... you look kinda fun". From here you will get into a very fun conversation indeed. I should note that this kind of approach will only really work if you carry yourself with a lot of confidence. Also note that you should stick to what you say, and don't apologise for being too forward if she takes offense. She isn't really offended, so don't worry... it's only a little test to see if your balls are real or not, and to get defensive at this stage will be instant game over, so stick to your guns.
More on the confidence thing, if you're fidgety and can't maintain eye contact, or your tone of voice fluctuates noticeably, or anything else that conveys that you are unsure of yourself, this kind of approach probably isn't gonna work out too well. I don't know your style, but I'm a direct kinda guy and I hate wasting time. I'm a firm believer in that she either likes you or she doesn't, and you don't want to waste time beating arond the bush trying to find out which. So, if you're comfortable with direct approaches, this will serve you very well. If not, I'm sure the others will provide useful advice that will suit your tastes. Have fun!